Single and multiple pipetting devices are well known to the art. Typical are U.S. Pat. Nos. 3,430,628, 3,568,735, 3,572,552, 3,261,208, 3,807,235, 3,982,438 and 4,158,035. The principal problem in connection particularly with multiple pipettes relates to accurately controlling the desired amount of fluid to be aspirated into the pipette tubes and then discharged. Cost, of course, is another major factor. The favored solution in the prior art is to employ multiple pipettes with plungers. Such devices of necessity have a column of air between each plunger and the liquid when the liquid has been inspirated. This necessitates calibrating the device for each aqueous liquid used. Further, they involve making costly parts and are expensive to assemble.
It is also known to use a rubber diaphragm which is displaced within cavities of a predetermined dimension to provide a negative pressure for the aspiration of a liquid and then a positive pressure for its discharge. The rubber diaphragm devices have essentially the same deficiencies as the plunger devices.
Hydrophobic filters which freely pass air or other gases but require an elevated pressure for the introduction of an aqueous liquid into and through the filter are known to the art for use, for example, in filtering air or permitting air to escape from a liquid mass.
In accordance with this invention, the problems of the prior art have been solved by the employment of a hydrophobic filter to limit the amount of liquid which can be drawn into the pipette tubes while at the same time permitting the passage of air or other gas for the creation of the necessary negative pressure for aspiration and positive pressure for discharge. The invention is superior because it eliminates the air column and any need for calibration since the total volume of the pipette chamber measures the quantity of liquid inspirated. Also, costly parts have been eliminated.